The present invention relates to a solid-state image sensor.
Much efforts have been made and are being made to develop the solid-state image sensors which are compact in size and light in weight and consume less power. The conventional solid-state image sensors may be classified into two types; that is, an X-Y switching matrix device and an charge transfer device. In the matrix type, photo-sensor unit cells such as diodes are arranged in the form of a matrix and are operatively connected to scanning circuits consisting of field-effect transistors. The charge transfer device may eliminate external scanning circuits such as those incorporated in the matrix device. The matrix device has a distinct disadvantage in that the light reception area of each unit cell is so small that the photoconductor conversion efficiency is very low. The charge transfer device is disadvantageous in that each unit cell has a dual function of detecting the incident light and transferring (to the output gate) a pack of electrons representative of the amount of incident light so that its signal-to-noise ratio (S/N ratio) is very low. Furthermore, because of absorption of the incident light by the electrode (which may, e.g., comprise polycrystalline silicon), the sensitivity to blue light is low.
In order to overcome the above problems, there have been devised and demonstrated various methods. For instance, a photoconductor layer is formed over the matrix type or charge transfer type unit cells. The characteristics such as spectral response and the dark current characteristic are dependent upon the properties of photoelectric films or layers. Therefore, the techniques enabling the formation of a photoelectric film or layer having excellent properties over a semiconductor substrate upon which are formed unit cells or circuit elements must be developed.
The inventors devised the photoconductor which has a hetero-junction between ZnSe and Zn.sub.1-x Cd.sub.x Te. This photoconductor has an excellent spectral response over the whole visible light range and exhibits very satisfactory desired other characteristic, so that its application to the target of an image pickup tube had been extensively studied and experimental work has been conducted on this subject. The image pickup tube incorporating this noble photoconductor is now known as "new vicon" which is a trademark of Matsushita Electric Industrial Co., Ltd. The light image is made incident on the ZnSe layer, so that a substrate made of a transparent glass and a transparent electrode mainly consisting of SnO.sub.2 is formed under the ZnSe film or layer.
However, in the case of the solid-state image sensor of the type wherein the photoelectric film or layer is formed over a semiconductor or silicon substrate upon which are formed the charge transfer type or the matrix type unit cells as described above, the silicon substrate absorbs almost all of the visible light, so that the light image must be made incident on the film or layer of Zn.sub.1-x Cd.sub.x Te.
Alternatively, a film or layer of (Zn.sub.1-x Cd.sub.x Te).sub.1-y (In.sub.2 Te.sub.3).sub.y may be formed over a silicon substrate and a film or layer of ZnSE may be formed over the former film or layer. However, when this structure is subjected to a heat treatment in order to improve its photoconductive characteristics, a satisfactory stoichiometric composition of (Zn.sub.1-x Cd.sub.x Te).sub.1-y (In.sub.2 Te.sub.3).sub.y cannot be attained, resulting in the increase in dark current flowing across a junction. Thus, it is imperative to make the light image incident on the film or layer of Zn.sub.1-x Cd.sub.x Te. However, the response or sensitivity to the blue color of the prior art hetero-junction is so low when the light image is made incident on the film or layer of Zn.sub.1-x Cd.sub.x Te that its application to a color image pickup tube or the like has been impossible.
In order to ensure satisfactory properties and characteristics of the photoelectric film or layer formed upon a semiconductive substrate, the photoelectric film or layer must form an ohmic contact with part of the semiconductor substrate and a transparent electrode formed over the photoelectric film or layer must not cause the degradation of desired properties and characteristics of the film or layer. In the hetero-junction of the type described above, a SnO.sub.2 or In.sub.2 O.sub.3 film or layer is formed as an electrode for the ZnSe layer, but the SnO.sub.2 or In.sub.2 O.sub.3 layer cannot attain a satisfactory ohmic contact with the silicon substrate. Thus, there had been a need for the development of a suitable electrode which may establish a satisfactory ohmic contact with the silicon and will not cause any degradation of desired properties and characteristics of the hetero-junction of the type described.
When the hetero-junction of the type described is used as a target of an image pickup tube, no electrode is needed on the side of the Zn.sub.1-x Cd.sub.x Te film or layer because the electron beam functions as an electrode. However, in the case of applications of the hetero-junction of the type described above to other devices, an electrode must be formed over the film or layer of Zn.sub.1-x Cd.sub.x Te. Up to now silver paste had been used as an electrode, but it is not stable as an electrode and it is difficult to form an electrode in a desired shape with the silver paste. Furthermore, when the light image is made incident on the film or layer of Zn.sub.1-x Cd.sub.x Te, the transmission of the incident light through the silver paste electrode is so low that the applications of the hetero-junction of the type described had been very limited
When the light image is made incident on the film or layer of Zn.sub.1-x Cd.sub.x Te as in the case of the present invention, a second electrode which is formed over the hetero-junction must be transparent. However, up to now there had been no report at all concerning such a transparent second electrode. Various methods for forming transparent electrodes themselves had been devised and demonstrated. They are for instance (1) a spraying method; (2) CVD method; (3) an electron beam evaporation method; (4) a vacuum evaporation method; and (5) a sputtering method.
According to the spraying method, a film or layer of SnO.sub.2 is formed over a substrate which is maintained at high temperature, by spraying an aqueous solution of SnCl.sub.4. In the CVD method, a film or layer of SnO.sub.2 is formed over a substrate, which is maintained at high temperature, by a chemical reaction between O.sub.2 and SnCl.sub.4 which is in a gas phase. In the case of the electron beam evaporation method, the electron beam is used to heat, melt and evaporate SnO.sub.2 or In.sub.2 O.sub.3 in an oxygen atmosphere at 10.sup.-3 to 10.sup.-4 Torr so as to deposite SnO.sub.2 or In.sub.2 0.sub.3 over a substrate. As in the case of the electron beam evaporation method, according to the vacuum evaporation method, In or Sn is heated, melted and evaporated in an oxygen atmosphere at 10.sup.-3 to 10.sup.-4 Torr so as to form a film or layer of In.sub.2 O.sub.3 or SnO.sub.2 over a substrate. According to the sputtering method, a target consisting of In.sub.2 O.sub.3 or SnO.sub.2 is sputtered in a vacuum of 10.sup.-2 to 10.sup.-4 Torr by the DC or AC glow discharges so as to form a transparent electrode layer of In.sub.2 O.sub.3 or SnO.sub.2.
These methods have their own advantages and disadvantages and are selected depending upon the desired properties and characteristics of a transparent electrode to be formed and upon the properties of a substrate upon which is formed a transparent electrode. For instance, in the cases of the method (1)-(4) set forth again, the substrate itself is responsible for the chemical reaction which results in the formation of a transparent electrode so that the substrate must be maintained at a temperature higher than 200.degree. C. In the case of the method (5) or the sputtering method, a transparent electrode may be formed even when a substrate is maintained at low temperatures, but because of the impingement of ions against the substrate, surface damages result very often.